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I went to Paris a few months ago, and we went over Greenland and I spotted the aurora, and I was wondering what exactly causes this light to happen?
Question Date: 2004-03-24
Answer 1:

The earth's magnetic field funnels cosmic rays, solar wind, and other charged particles into the poles so that they strike the upper atmosphere. The energy released by the impact of these particles and interaction with the air emits light - the aurora.

Answer 2:

What you saw was an aurora borealis (in the southern counterpart it is called aurora australis). It is interesting that we see the aurora close to the north and south pole here on Earth, but the aurorae actually starts at the center of the solar system.

The Sun is constantly blowing out a stream of protons and electrons, called the Solar Wind. It takes about three days for those particles to get near the earth and interact with the Earth's magnetic field. You can think of the Earth's magnetic field as coming out of the ground at the north pole, going around the earth from north to south, and then going back in at the south pole.

The particles that hit the magnetic field want to 'slide down' the field to the poles. When they do this, they eventually hit the earth's atmosphere. These particles are moving very fast, and when they hit the air they slam into air atoms that make up our atmosphere, sometimes knocking loose more electrons. After a short time these free electrons get back together with their parent atoms, and when they do, the atoms give off light. This light is usually greenish, but is sometimes red as well (it depends on what kind of atom you have). When this happens to lots of atoms, you get lots of light--the aurorae. The amount of energy involved can sometimes be more than the entire United States uses!

From Philip Plait, Astronomer who answered that question at: here.

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